The quality of the brass is the factor,not the action. Steel does not get brittle or other internet baloney with age.
Due to the brass itself you want to to keep the loads around 55,000 PSI for no other reason than for case life. You could go hotter but you will prob blow primers. With the small bore of the 6.5 some powders can and will spike the pressure if too much is used.
I have a Tweed Carbine that I built and it handles my loads well with no bolt stiffness,a sign of a too high pressure load.

Tin Can Sailor
'Nam Vet
NRA Endowment Member
President New Mexico Military Surplus Rifle Pistol Shooters

The factory hardening certainly DOES effect things and age effects everything.
The case is not a factor unless you exceed the strength of the web and case head to hold pressure (or there is a feed ramp that encrouches further up the case).
The MAX load would be the proof load, if the metallurgy has not changed with age, but that is a DANGEROUS view to take.
Find a load it likes and enjoy it.

Sorry,you need to do more research. I have been shooting and building firearms for over 50 years as well as serving in the Navy as an Ordnance Chief. It was my job to test weapons until they failed. What it took to so called " blow up a weapon" was far more than the average Joe knows about.
I recommend all Read "Hatchers Notebook" as well as Dr.Mann's book " A bullets flight". Both talk about firearms strength and both did extensive testing of them.
Unless a firearm has been in a fire, fired hundreds of very hot loads or very poorly stored,the action will be as strong as the day it was made.
I have never seen an action fail due to age and my club and myself fire thousands and thousands or rounds through rifles from the 1880's on each year.

Tin Can Sailor
'Nam Vet
NRA Endowment Member
President New Mexico Military Surplus Rifle Pistol Shooters